Generally, an engine includes a cylinder block and a cylinder head block. In the cylinder block, a cylinder is formed, and in the cylinder head block, an intake port and an exhaust port are formed. Further, in the cylinder head block, an injector and a glow plug are provided. That is, air is introduced into a combustion chamber through the intake port and fuel is injected by the injector, so that the air and the fuel are mixed to form a mixed gas, and the mixed gas is exploded and burned by a compression stroke of a piston. After being burned, the mixed gas is discharged as an exhaust gas through the exhaust port.
Hereinafter, referring to FIG. 1 to FIG. 3, a conventional engine intake port structure will be described.
FIG. 1 is a diagram provided to explain a conventional engine intake port structure. FIG. 2 is a bottom view of a cylinder head block provided to explain the conventional engine intake port structure. FIG. 3 is a longitudinal cross-sectional view of an intake port provided to explain the conventional engine intake port structure.
As illustrated in FIG. 1, an intake manifold 1 is provided on one side of a cylinder head block 3. The intake manifold 1 is configured to suck fresh air from the outside or to be supplied with intake air by a turbocharger. The intake manifold 1 and a combustion chamber 4 are connected to each other by first and second intake ports 2a and 2b. To be more specific, in the cylinder head block 3, an intake port 5 and an exhaust port 7 are formed, and the intake port 5 and the exhaust port 7 are disposed within the combustion chamber 4.
The intake port 5 is connected to the above-described first and second intake ports 2a and 2b, and a valve unit 8 is installed. That is, if the valve unit 8 is opened at the intake port 5, fresh air is supplied into the combustion chamber, and if the valve unit 8 closes the intake port 5, the combustion chamber is sealed.
Meanwhile, a valve sheet 6 is provided at the intake port 5. When the valve unit 8 is closed, the valve sheet 6 is brought into close contact with the valve unit 8 so as to maintain a sealed state of the combustion chamber.
Further, after a mixed gas including air and fuel is burned, an exhaust port 7 is configured to exhaust the gas. Although not illustrated in detail, the exhaust port 7 is provided with an exhaust valve for opening/closing the exhaust port 7.
Furthermore, in the cylinder head block 3, an injector 9 is installed at a central portion of the combustion chamber 4. The injector 9 is configured to inject fuel to the combustion chamber.
Also, in the cylinder head block 3, a glow plug 10 may be installed so as to be close to the injector 9. The glow plug 10 is used to warm up fuel.
Meanwhile, intake air may be supplied to the intake manifold 1. Such compressed air may have a swirl depending on a shape of a flow path while passing through the first and second intake ports 2a and 2b. The swirl enables the intake air and fuel to be mixed well in the combustion chamber.
Meanwhile, as illustrated in FIG. 3, in the conventional engine intake port structure, a corner at an end of the intake port 5 may be formed in a linear fashion without a particular shape, or even if a chamber is formed, the chamber is provided in a concentric manner with an inner diameter of the intake port 5.
Therefore, when the valve unit 8 is opened at the intake port 5, opening widths w1 and w2 are equal to each other at four sides of the valve unit 8 regardless of a position of a level of the valve unit 8.
Meanwhile, at the time when the valve unit 8 is opened, the opening widths w1 and w2 are narrow, and thus, the intake air is introduced into the combustion chamber at a high pressure while the swirls are formed in the intake air. The intake air introduced as such flows and is mixed with fuel.
Further, a glow plug 10 may be provided in the combustion chamber 4. That is, the intake air may be supplied toward a space where the glow plug 10 is disposed. Thus, the intake air may cause interference with the glow plug 10 and the swirl may be randomly scattered, so that the intake air and the fuel may not be mixed well.
Otherwise, as one of important considerations in developing a combustion unit of an engine, a vortex flow, i.e., a swirl flow, of air sucked into a combustion chamber is important. This is because only when the swirl flow of the intake air is big enough, the air and fuel can be smoothly mixed and smoke can be reduced.
As components for generating a swirl, there are the first and second intake ports 2a and 2b. The first intake port 2a may be provided as a helical port, so that a swirl is generated while the intake air passes through the first intake port 2a. Further, the second intake port 2b is disposed such that the intake air can be introduced in a tangential direction from an edge of the combustion chamber, and, thus, the intake air forms a swirl itself while passing through the second intake port 2b. 
However, as described above, in order to generate swirls by the first and second intake ports 2a and 2b, an air volume over a certain amount needs to be secured. For this reason, a swirl (vortex) flow ratio is proportional to an opening width of an intake valve. That is, if the opening width of the intake valve is small, the swirl flow ratio is low, whereas if the opening width of the intake valve is large, the swirl flow ratio is high.
Therefore, the swirl flow ratio increases as the opening width of the intake valve increases in the middle of an intake stroke. However, toward the end of the intake stroke, as the intake valve is closed, an air volume decreases and a swirl of air sucked through an intake port becomes gradually smaller. Further, if the length of time during which air remains in a combustion chamber is short due to a swirl flow, there may be a problem of incomplete combustion.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.